INTERACTIONS OF NICKEL(II) WITH ADENOSINE, URIDINE AND CYTIDINE MONOPHOSPHATES - A CALORIMETRIC STUDY

The thermodynamic parameters of the interaction between nickel(II) and 3'AMP, 2'AMP, 5'UMP and 5'CMP in aqueous solution were determined cal orimetrically (ionic strength adjusted to 0.1 with tetramethylammonium bromide, 25 degrees C at physiological pH). The experimental conditio ns were carefully selected to avoid polynuclear complex formation. The calorimetric data confirm the existence of the [(Ni(NMP)(2)](2-) comp lexes for 3'AMP and 2'AMP, the latter previously studied by Frey and S tuehr by potentiometry. The [(Ni(NMP)(2)](2-) complex formation consta nt (log K-2 = 1.96 and 2.16 for the 3'AMP and 2'AMP derivatives, respe ctively) and complex formation enthalpies (Delta H-1 = +4.5 kJ mol(-1) and Delta H-2 = -10.0 kJ mol(-1) for the 3'AMP complexes, and Delta H -1 = +2.4 kJ mol(-1) and Delta H-2 = -22.3 kJ mol(-1) for the 2'AMP co mplexes) were determined. The values of Delta H-1 for the complexes Ni (5'UMP) and Ni(5'CMP) are, respectively, 8.3 and 8.7 kJ mol(-1). The d egree of macrochelation indicated by Sigel after very careful potentio metric studies is confirmed by the calorimetric data, and follows the order: [Ni(5'AMP)](cl) > [Ni(2'AMP)](cl) > [Ni(3'AMP)](cl). These resu lts confirm the formation of the ML(2) complex in solution and agree v ery well with the hypothesis of 'stacking' between the purine bases pr omoted for metal ions.